As is widely known, in recent years, as photo-curable resin compositions for use in printing ink, paint, a coating material, and the like, those utilizing a cationic photopolymerization reaction have attracted attention as well as those utilizing a radical photopolymerization reaction. A photo-curable resin composition utilizing a cationic photopolymerization reaction contains, as main components, a cationic photopolymerizable compound, which contains, for example, an epoxy group, an oxetane group, a vinyl ether group, etc., and a cationic photopolymerization initiator for the polymerization of such a compound. The cationic photopolymerization initiator plays a role in generating protons to initiate a cationic photopolymerization reaction. Well known examples thereof include sulfonium salt based cationic photopolymerization initiators and iodonium salt based cationic photopolymerization initiators.
A cationic photopolymerization initiator is required to have such characteristics that, for example, it efficiently absorbs light, the bond between the cation atom and the carbon atom of the substituent (e.g., S+-Ph bond or I+-Ph bond) is easily cleaved, and the cation atom is susceptible to reduction. Some of the already commercially available cationic photopolymerization initiators are excellent in these characteristics, but there is a demand for a cationic photopolymerization initiator with further improved characteristics.